In recent years plastic has replaced glass as a preferred material for the formation of spectacle lenses. Plastic lenses are lighter, are tintable, and in the case of polycarbonate for example, possess the ability to absorb impact without fracturing. Higher index plastics permit the formation of thinner, lighter lenses and yet generally maintain a convex sphere as the choice for the front surface.
Apparatus of the prior art for injection molding ophthalmic lenses from a suitable plastic such as polycarbonate resin, for example, employs a concave insert used as a negative to produce finished convex surfaces of the lenses made on the apparatus. With the advent of rigid polishing equipment, purer slurries and better accuracy on the preform, fabrication of spherical mold inserts has been simplified. These inserts or "tools" may be fabricated from glass, stainless steel, germanium, aluminum and various other materials.
Convex inserts may be fabricated in a similar manner to produce the concave surface of the finished lens.
Multi-focal molded lenses generally have the reading or "near" surface power on the convex surface of the lens. Mold inserts for forming the required convex surface are made in various ways from glass, metal or some other suitable substrate.
The ability to fuse glass makes it perhaps the easiest to use in the formation of a spherical segmented multifocal mold insert. In making the concave bifocal mold component, a glass concave blank is prepared with ground and polished concave and convex surfaces. Next, a second spherical radius is countersunk into the existing distance radius of the concave surface of the blank. This second radius is the final reading radius of the mold component. Then a glass "button", having a convex radius matching the concave countersink, is placed in proper orientation and the assembly is then fused in a kiln making the two components one. After fusing, the distance spherical radius is once again regenerated and polished to bring the "add" portion down to the proper dimension.
In the manufacture of concave mold inserts, it has also been suggested that a spherical concave surface of one radius might be generated on a second previously existing concave radius.
"D-Seg" multi-focal lenses and "Executive" multi-focal lenses in which the near vision portion extends completely across the lower portion of the lens, may be made from a one-piece glass or metal component by another technique which requires no fusing and which allows placement of the reading portion directly into the concave surface of the component or insert. Since the surface to be produced is not symmetrical, these inserts require special stock removal and polishing techniques. Creation of a small circular multi-focal segment into a larger concave sphere of another radius, as described earlier, is relatively simple because the segment is round and spherical or symmetrical. However, a "D-Seg" or "Executive", not being symmetrical, does not allow for rotation of any secondary finishing devices such as a round convex spherical lap or disc generator. Special toric polishing equipment is necessary to fabricate inserts of this type. Electrical Discharge Machines (EDM's) using a graphite electrode allow for rapid stock removal from a conductive substrate to facilitate formation of the second concave radius providing the reading section onto the first concave radius providing the distance section. Platings are applied before final finishing to ensure a finely polished surface with minimal optical distortion.
In all instances the reading concave radius is generated into the larger distance concave radius to a dimension somewhat larger than the final dimension. This is done in order to permit the reading portion to be protected by some type of covering and then the distance concave spherical radius be reworked to reduce the size of the reading portion to within a specific tolerance. Owing to the fact that the distance portion is spherical and symmetrical, this may be easily accomplished with a number of techniques.
Owing to their cosmetic appeal, plastic progressive multi-focal lenses have come into wide use. Progressive multi-focal surfaces are non-symmetrical and the concave mold inserts used to manufacture these lenses require special fabrication techniques. In one such technique a ceramic block first is cut to a predetermined concave geometry. Next, a symmetrical glass blank, also of a specific geometry, is then placed on the ceramic block and the assembly is put into a kiln. The temperature within the kiln is raised and lowered to a specific temperature cycle to slump the glass so that the back or convex surface of the glass blank comes into contact with the concave surface of the ceramic. In the course of the temperature cycle, a form of the image of the ceramic blank is transferred through the glass blank onto the concave surface thereof. This is the conventional technique for producing glass concave progressive mold inserts.
The quest for lenses which are even thinner and lighter resulted in the development of aspheric finished and semi-finished lenses. Such lenses are manufactured using concave molding components or inserts which are rotationally symmetrical and yet not spherical. Since spherical polishing equipment cannot be used to form these inserts, they are slumped as is done to produce the progressive multi-focal mold components or they are polished by computer-aided polishing equipment using special programs. A third method of generating the aspheric concave surface is through single point diamond turning. Usually, a high quality stainless steel is chosen as the substrate and, after machining, the concave surface is plated with approximately 0.007" of nickel and then retrued using a single point diamond turning center. After turning the concave surface need only be hand polished with fine grain diamond compound.
It will readily be appreciated that in the casting injection or injection/compression molding process, the original concave mold component used to produce the convex portion of the finished lens is used over and over again. The durability of the insert permitted successful image transfer over only a limited number of cycles. Nickel replication, which is a system of making duplicate copies of an original mold component, permits many copies to be made from a master insert. These copies can then be used as concave mold components or inserts to produce convex lens surfaces.
The advent of aspheric base curves placed on the convex surface of finished ophthalmic lenses, has given rise to a demand for a new style multi-focal. While the new multi-focal is quite similar to the "D-seg" shaped multi-focal discussed hereinabove, a critical difference is that the new style "D-seg" has an aspheric distance portion and a spherical reading portion whereas the distance and reading portions of the original multi-focal both were spherical.
It will be remembered that fabrication of the original multi-focal discussed hereinabove used spherical and toric polishing equipment on the concave distance and reading portion spheres.
Concave aspheric multi-focal inserts cannot be manufactured using any of the techniques discussed hereinabove. One possible method which was considered is a variation of the method of forming a spherical multi-focal concave mold insert. In this method the concave aspheric distance portion first is accurately roughed into the face of the stainless steel mold component. The component is then heat treated. Next, it is set up in the Electrical Discharge Machine with a graphite electrode having the appropriate spherical convex form located above the component and normal to the reading portion optical position on the aspheric surface. The "D-seg"-shaped reading portion is then formed in the concave aspherical surface. The "D-seg" is then roughed and polished on the toric polisher to a predetermined size. Then cast iron laps, polishing pads, and various grades of alumina oxide and diamond are used to prepare the "D-seg" for plating. The entire aspheric concave surface then is plated with approximately 0.007" of nickel by an electroless plating process. The "D-seg" is then ground and polished to a predetermined size which is slightly larger than the final size. The insert is then mounted in the single point diamond lathe and the aspheric surface is generated. As the nickel plating is removed in the course of this operation, the "D-seg" is reduced in overall size. When the "D-seg" is of the proper size, the machining operation is complete. The aspheric surface may then be hand polished.
While the technique just described is possible, it is so expensive as to be prohibitive. In a typical matrix, eighteen mold inserts are required to cover each base curve. Many base curves are employed to cover a range of prescriptions.
In addition to the necessity for producing a multiplicity of inserts, the success rate for the completion of each mold insert also is low. While the electroless plating is rapid and produces a uniform deposit, it often has pits. Further, breaking through the nickel surface for damage occurring during any of the various steps of the fabrication process, could result in failure.